This invention relates to microwave power sensors and specifically to a power sensor which utilizes diodes to sense the power of microwaves that have frequencies near and above the resonant frequency of the diodes.
Microwave power sensors have a number of applications in fields ranging from communications to national defense. Power sensors using diodes as the sensing means have been used to measure microwave power for a number of years, but the power sensors of the past could only be used to measure the power of waves having frequencies much below the resonant frequency of the diodes.
To illustrate the concept of a resonant frequency, a commonly accepted model of a diode formed on a semiconductor substrate is shown in FIG. 1. The diode itself is represented by a junction capacitance C.sub.j and a non-linear resistance R.sub.o. The spreading resistance R.sub.s is the resistance encountered by the current as it flows through the bulk of the semiconductor substrate. The lead inductance L.sub.i is the inherent inductance of the electric connection to the diode, and the parasitic capacitance C.sub.p is the capacitance associated with the semiconductor substrate itself found between the two terminals A and B. Due to the reactive elements C.sub.j, L.sub.i, and C.sub.p, there is a certain frequency at which the diode will resonate, and this resonant frequency is determined by the values of these elements. Even with today's fabrication technology, it is only possible to produce diodes having resonant frequencies of as high as about 60 GHz. The resonant frequency of a diode is important because it determines the frequency range within which the diode may usually be used as a power sensor.
A typical power sensing circuit using a diode is depicted in FIG. 2 comprising a matching load resistor R.sub.m, a power sensing diode D, and a very large output capacitor C.sub.o, the capacitor C.sub.o having a negligible RF impedance. A qualitative plot of the DC output of this sensing circuit against frequency is provided in FIG. 3 for input waves having the same power but different frequencies. For input waves having a frequency much below the resonant frequency of the diode, the impedance of diode D is much greater than R.sub.m so that the input wave 20 encounters a parallel combination of the matching load resistor R.sub.m and a virtual open circuit due to the large impedance of the diode D. As a result, most of the power of the wave 20 is absorbed by the matching resistor R.sub.m. The diode D detects the voltage of the input wave 20 at node N.sub.1 and rectifies it to produce a DC signal across output capacitor C.sub.o which has an amplitude proportional to the square of the voltage across terminals T.sub.1 and T.sub. 2. Since power is proportional to the square of the voltage, the amplitude of the DC output 22 is thus proportional to the power of the input wave 20. For this reason, the diode is said to be operating under a square law regimen. The DC output curve of the circuit of FIG. 2 for input waves having frequencies much below the resonant frequency of the diode D is depicted by curve portion 24 of FIG. 3. For frequencies below frequency F.sub.L, the DC output for input waves having the same power is constant, meaning that there is no frequency dependence. The flatness of curve 24 is quite desirable since the sensor is a power sensor, not a frequency sensor. Waves having the same power should cause the sensor to output the same signal regardless of the frequency of the wave.
For frequencies above F.sub.L, however, the DC output is no longer frequency independent as shown by curve 26. This is due to the fact that the diode D of FIG. 2 in reality behaves as the circuit of FIG. 1, which should now replace D in FIG. 2. The DC voltage that appears across the output capacitor C.sub.o really is proportional to the square of the fraction of the input RF voltage wave that appears across the parallel combination of C.sub.j and R.sub.o. For frequencies above F.sub.L the diode D behaves very nearly as a series resonant circuit, the resonance being caused mainly by the inductance L.sub.i and the junction capacitance C.sub.j. As frequency increases and approaches the resonant frequency of the diode, the voltage across the junction capacitance C.sub.j increases due to the resonance effect, and the DC voltage across the output capacitor C.sub.o, which is proportional to the square of the voltage across C.sub.j, also increases. Another effect is that, as the frequency of the input signal increases and approaches the resonant frequency of the diode, the diode impedance decreases, causing the parallel combination of its impedance with that of the load R.sub.m to present a mismatched load to the input wave. This process continues as the frequency of the input continues to increase, until the resonant frequency F.sub.R is reached. At this frequency F.sub.R, maximum DC output signal across the output capacitor C.sub.o occurs. The diode impedance reaches a minimum at frequency F.sub.R. As the frequency of the input signal continues to increase, the RF voltage across the diode junction capacitor C.sub.j starts to drop, and consequently, so does the DC voltage across the output capacitor C.sub.o. The DC output of the sensor quickly falls off with increased frequency, as shown by curve portion 28 of FIG. 3. The diode overall impedance begins to rise for frequencies above F.sub.R. In regions 26 and 28, the sensing circuit of FIG. 2 is outputting different DC signal levels at different frequencies even though the power of the input wave is constant. This frequency dependence is quite undesirable. A flat response like that of curve 24 would be more preferable, if not essential. Due to this frequency dependence, diodes have seldom been used as power sensors at frequencies near or above their resonant frequencies. As previously mentioned, it is only possible, with current technology, to produce diodes having resonant frequencies of up to about 60 GHz. However, there are many applications which require a power sensor capable of operating at frequencies way above 60 GHz (e.g., between 75 and 110 GHz). Therefore, a need exists for an apparatus which would allow a diode to be used near and above its resonant frequency to measure accurately RF power.
Thus, it is an object of the invention to provide a power sensor which employs a diode to measure accurately the power of electromagnetic waves having frequencies at, near, or above the diode's resonant frequency.
Another object of the invention is to provide a broad-band power sensor which has a relatively flat frequency response up to between 75 and 110 GHz and above.
Yet another object of the invention is to provide a power sensor which has a load impedance which matches the characteristic impedance of the input waveguide.
SUMMARY OF THE INVENTION
In accordance with the objects of the invention, a broad-band electromagnetic wave power sensor is provided which is capable of operating at frequencies near and above the resonant frequency of the sensing diode. The power sensor of the invention comprises a sensing diode, an input wave conditioning means, and a waveguide. The sensing diode of the invention is a diode of regular construction having frequency characteristics such that, for input waves having frequencies near or above its resonant frequency, the ratio of the DC output of the diode to the power of the input wave (DC output/power input ratio) is frequency dependent.
The conditioning means, adapted to receive an input electromagnetic wave and output a conditioned wave having a fraction of the power of the input wave, is designed in such a way that it offsets the frequency dependence of the diode. This may be achieved by designing the conditioning means to transmit a greater fraction of the input wave to the diode at frequencies where the DC output/power input ratio of the diode is below a reference value. Likewise, the conditioning means transmits a lesser fraction of the input wave to the diode at frequencies where the DC output/power input ratio is above a reference value. In this manner, the DC output of the diode may be kept relatively constant for input waves having equal power but different frequencies. The conditioning means is further adapted to have a load impedance which substantially matches the characteristic impedance of the input electromagnetic waveguide so that the conditioning means minimizes the reflection of the input wave. This allows the power of the wave to be measured more accurately. Furthermore, the conditioning means is adapted to attenuate the input signal to a sufficient degree to ensure that the sensing diode functions within the square law region.
The waveguide of the invention receives the conditioned wave outputted by the conditioning means and conveys it to the sensing diode. The sensing diode responds to the conditioned wave by outputting a DC signal which is proportional to the power of the conditioned wave. The sensing diode is preferably attached to the waveguide at a point where a voltage maximum of the conditioned wave always occurs so as to improve the sensitivity of the sensing diode across a broad frequency band.
By combining the conditioning means, waveguide, and diode into a single system, a power sensor may be produced which is capable of providing very nearly constant DC output signals for input waves having equal power but different frequencies. Thus, a broad-band power sensor is provided which may be used to measure the power of electromagnetic waves having frequencies near and above the resonant frequency of the sensing diode.